1. Field of the Invention
The present invention relates to a semiconductor device including an electric fuse.
2. Description of the Related Art
In recent years, there has been proposed a novel method of disconnecting an electric fuse, which is called a crack assist type method. In this method, the structure of the electric fuse, how to apply a voltage to the electric fuse, and the like are controlled, to thereby forcibly cause a conductive material forming the electric fuse as a part thereof to flow outwardly, that is, into an insulating film located around the conductive material, during disconnection of the electric fuse. As a result, a movement and supply balance among the materials is lost. In this manner, a large disconnection point is formed in another part. Accordingly, the possibility that the disconnected electric fuse is connected again may be greatly reduced, and the disconnection state may be satisfactorily maintained (see, for example, JP 2007-305693 A).
FIG. 5 is a plan view illustrating a structure of a semiconductor device 50 including an electric fuse 10 that is similar to the electric fuse described in JP 2007-305693 A. The electric fuse 10 includes a terminal 14 and a terminal 24, an upper layer fuse interconnect 12 connected to the terminal 14, a lower layer fuse interconnect 22 connected to the terminal 24, and a via 30 that connects the upper layer fuse interconnect 12 and the lower layer fuse interconnect 22.
FIGS. 6A to 6C are cross-sectional views taken along the line C-C′ of FIG. 5. The semiconductor device 50 has a structure in which an interlayer insulating film 202, an etching stopper film 204, an interlayer insulating film 206, and an interlayer insulating film 210 are laminated on a substrate (not shown) in the stated order. The terminal 24 and the lower layer fuse interconnect 22 are formed in the interlayer insulating film 202, whereas the via 30, the upper layer fuse interconnect 12, and the terminal 14 are formed in the interlayer insulating film 206.
In the electric fuse 10 having the above-mentioned structure, when a voltage is applied between the terminal 14 and the terminal 24, a current is caused to flow in a direction from the lower layer fuse interconnect 22 to the upper layer fuse interconnect 12 (FIG. 6A). As a result, the via 30 and the upper layer fuse interconnect 12 are heated. Further, the conductive material such as copper, which forms each of the via 30 and the upper layer fuse interconnect 12, expands. A via diameter of the via 30 also expands to be larger than an original via diameter thereof (FIG. 6B). After that, when the expansion of the upper layer fuse interconnect 12 advances to a certain degree, a crack occurs in the interlayer insulating film 206 located around the upper layer fuse interconnect 12. As a result, the conductive material of the upper layer fuse interconnect 12 flows into the interlayer insulating film 206, to thereby form a flowing-out portion 70 (FIG. 6C). In a case where the electric fuse 10 is normally disconnected, the conductive material of the via 30 also moves along with the flowing-out of the conductive material of the upper layer fuse interconnect 12, with the result that a disconnection point is formed in the via 30.
Incidentally, the inventor of the present invention found that the following problems arise when the electric fuse having the structure as described in JP 2007-305693 A is to be disconnected by the crack assist type method.
FIGS. 7A to 7C are cross-sectional views illustrating the upper layer fuse interconnect 12, the via 30, and the lower layer fuse interconnect 22 included in the electric fuse 10 of FIG. 5. In the electric fuse 10 having the structure as illustrated in FIG. 5, when a voltage is applied between the terminal 14 and the terminal 24, more heat is generated in a portion between the terminal 14 and the terminal 24, the portion being higher in resistance, and thus the temperature of that portion becomes the highest in the electric fuse 10. Further, the terminal 14 and the terminal 24, each having a large area, function as heat sinks. Therefore, a point at which the temperature becomes the highest in the electric fuse 10 during the disconnection of the electric fuse 10 is somewhere around the portion having a large heat generation amount, which is away from the terminal 14 and the terminal 24. Further, in disconnecting the electric fuse 10, a position at which the flowing-out portion 70 is formed may vary, depending on areas or the like of the respective structural components.
In order that the flowing-out portion 70 is formed in the upper layer fuse interconnect 12 and that the disconnection point is formed in the via 30, a width and a length of each of the upper layer fuse interconnect 12 and the lower layer fuse interconnect 22 are appropriately adjusted. As a result, a temperature near the connection point of the upper layer fuse interconnect 12 with the via 30 may be set to be the highest in the electric fuse 10 (FIG. 7A). However, during a manufacture of the semiconductor device 50, there is a case, for example, as illustrated in FIG. 7B, in which a film thickness of the upper layer fuse interconnect 12 reduces from a preset value d1 of FIG. 7A to a value d2 of FIG. 7B (d1>d2), due to manufacturing fluctuations such as fluctuations in film formation or polishing. In such a case, a resistance of the upper layer fuse interconnect 12 changes to become higher, with the result that the point at which the temperature becomes the highest during the disconnection shifts to a center part of the upper layer fuse interconnect 12, which is away from the via 30.
As described above, when the point at which the temperature becomes the highest during the disconnection is shifted away from the via 30 to excessively approach the center part of the upper layer fuse interconnect 12, the via 30 is not sufficiently heated. As a result, a disconnection failure that the via 30 is not disconnected occurs. Description is given with reference to FIGS. 8A and 8B. FIGS. 8A and 8B are cross-sectional views taken along the line C-C′ of FIG. 5, and illustrate steps subsequent to the steps of FIGS. 6A to 6C during the disconnection of the electric fuse 10. When the upper layer fuse interconnect 12 flows out before the conductive material that forms the via 30 is sufficiently heated to be melted enough to be movable, a void is formed in the upper layer fuse interconnect 12, and then a disconnection point 72 is formed in the upper layer fuse interconnect 12 (FIG. 8A). In this case, the conductive material of the via 30 does not move. After that, the conductive material continues to thermally contract until the temperature thereof reaches room temperature, and then solidifies while leaving the disconnection point 72 above the via 30 and in the upper layer fuse interconnect 12 (FIG. 8B). However, in the structure described above in which the disconnection point 72 is formed in the upper layer fuse interconnect 12, there is a fear that the electric fuse 10 is reconnected in a case where the conductive material coagulates and then deforms due to thermal history thereof in the following assembly process or the like of the semiconductor device 50 or in actual use of the semiconductor device 50 under high temperature.
Further, there may be another case, for example, as illustrated in FIG. 7C, in which the film thickness of the upper layer fuse interconnect 12 increases from the preset value d1 of FIG. 7A to a value d3 of FIG. 7C (d3>d1). In such a case, the resistance of the upper layer fuse interconnect 12 changes to become lower, with the result that the point at which the temperature becomes the highest during the disconnection shifts to a bottom part of the via 30. However, when the point at which the temperature becomes the highest during the disconnection approaches the bottom part of the via 30 as described above, the conductive material flows out also from the bottom part of the via 30, and may be short-circuited with the conductive material that has flowed out from the upper layer fuse interconnect 12.